Most rotary shredding devices of the type disclosed are driven directly by an electric motor through an appropriate speeding reducing transmission. Although such electric drive motors may be provided with overload protection or a reversing circuit, they, nevertheless, tend to burn out when subjected to sudden excessive torque demands, such as when the cutters are jammed with material too hard or too large for them to handle. Because of this deficiency in direct drive electric drives for such rotary shredders, shredders have also been equipped with hydraulic motors using electric motor driven hydraulic pumps to supply fluid pressure to such hydraulic motors. In this way, the hydraulic motor circuit can be designed with pressure relief valves to prevent excessive operating pressures and in this way, isolate the electric pump motor from excessive torque loads.
An example of such hydraulic drive is shown in U.S. Pat. No. 4,034,918, which describes a hydraulic driven dual shaft shredder. The shredder of the 4,034,918 invention is meant to shear or shred the material during one direction of rotation only. Should an overload "jam" condition occur, a pressure switch in the hydraulic circuit of the hydraulic motor is triggered to reverse the rotation of the shafts and thereby clear the jam. After a suitable time period, the shredder again will begin rotating in the forward direction to shred material.
Several shredders have been devised which use only a single rotary shaft working in conjunction with fixed blades or shear bars. One example of a single shaft rotary shredder is disclosed in U.S. Pat. No. 4,394,983 to Ulsky. The Ulsky patent describes a single rotary shaft having blades mounted in a side-to-side relationship down the shaft. Spaces between successive blades allow replaceable fixed rack teeth to fit between the blades. A mechanical linkage allows these blades to swing out of the way in case of an overload condition.
A single shaft rotary chopper is disclosed in U.S. Pat. No. 4,600,158 to Matoba. Again, cutting blades are spaced along a rotary shaft. Cooperating fixed blades are mounted in the gaps between each rotary blade to assist in chopping the material. No overload provisions are provided.
Accordingly, there is a need for a hydraulic drive for a rotary shredder to prevent the burning out of electric motors and furthermore, a hydraulic drive arrangement with a reliable and automatic reversing control to reverse the cutting blades to allow shredding in both directions of rotation of a single rotary shaft to equalize wear and to reverse the direction of rotation upon detection of an overload condition.